Stent-grafts are tubular constructions for use inside blood vessels. They generally comprise two components: a quasi blood-tight tube that is usually formed of textile or membranous material, and a reinforcing structure which is usually made of wire or other filamentous metallic material.
The tubular graft component is usually made from woven polyester fabric, although a few designs employ PTFE membranes. The graft is supported on metallic rings which are generally formed from wire but are occasionally cut from metal tube by means of laser cutting or similar means. When attached to the graft, the metal rings can define a single plane at an angle to the axis of the graft, the rings can be corrugated so that they define the surface of a short cylinder on the surface of the graft, or the rings can be fenestrated or linked so as to produce a long cylindrical reinforcing element on the surface of the graft.
In almost all designs, the reinforcing rings are attached to the surface of the graft by means of sutures which are generally applied by hand. Some designs employ many hundreds or thousands of sutures and the time and cost associated with attaching these sutures is high. Additionally, the burden of assuring the quality of every stitch is expensive, while the implications of poor quality stitching in a device which is implanted into a patient can be particularly serious to the health of the patient.
There is also a requirement to manufacture stent-grafts to fit the anatomies of individual patients. In particular, the lengths, diameters, tapers, secondary tapers and the location of side-branches of stent-grafts are required to be tailor-made for each patient and this can take an unfeasible period of time when the stent-graft is made by hand.
An alternative solution has been described in WO 99/37242 (in the name of the present applicant) in which computerised embroidery is used to manufacture a flat-form device which is subsequently rolled into a tube. This approach solves many of the issues associated with hand manufacturing but results in a seam and prevent some continuous structures from being designed.
GB 2165559 (University College London) discloses a sewing machine for forming stitches in a substrate, for example body tissue, during surgery. The sewing machine employs suction to pull a folded section of the substrate into the machine so that it is disposed between a needle and a hook. The needle can then be used to feed thread through the folded section of substrate to emerge the other side, and to engage the thread on the hook. This action is repeated with the sewing machine being moved along the substrate, thereby forming stitches in the substrate. This sewing machine could not be employed to stitch thread to the wall of a tubular graft, because the graft would not be sufficiently compliant to enable a folded section of graft to be sucked into the machine.
U.S. Pat. No. 4,502,159 (Shiley Incorporated) discloses a method for forming a tubular prosthesis by rolling pericardial tissue into a tube and stitching along the tube to form a longitudinal seam. However, the stitches are formed conventionally by passing a thread from one side of the seam to the other on the outside of the tube.
U.S. Pat. No. 4,241,681 (Porter) discloses a sewing machine for sewing a series of spaced reinforcing rings on a long flexible tube of fireproof fabric. The machine comprises, a long tubular support over which the work piece is pulled like a sleeve on an arm. A fixed stitching mechanism is provided for forming chain stitch in the work piece, and a puller mechanism advances the work piece over the support as the stitches are formed therein.
U.S. Pat. No. 4,414,908 (Janome Sewing Machine Co., Limited) discloses a suturing machine for suturing incised parts of a patient. The machine comprises a needle holder (effectively a pair of pliers) and a shuttle holder which is slideably mounted on the needle holder. This means that movement of the needle independently of the shuttle is not possible for all degrees of freedom. In particular, movement of the needle holder to pierce the needle through the wall of a tubular implant would inevitably result in a corresponding movement of the shuttle within the bore of the implant. In practice, it would not be possible to manipulate the needle without causing the shuttle to impact the side walls of the implant, thereby risking damage to the implant.
In an alternative embodiment of U.S. Pat. No. 4,414,908, the shuttle is mounted on a rod which is slideable parallel to the longitudinal axis of the needle holder and the needle holder is rotatable circumferentially about the axis of the shuttle rod. This means that rotation of the needle holder causes the needle to follow a circumferential path which is a constant distance from the shuttle. Thus the needle could never penetrate the side wall of a tubular implant. Rather, it would simply circle around the implant, keeping a constant distance from the centre of the bore.